Hong Kong police have arrested two elderly sisters suspected of running a prostitution ring after raiding their luxury Mid-Levels home, believed to have served as a 24-hour call centre for the syndicate.

Officers from the Organised Crime and Triad Bureau raided the flat – measuring more than 1,000 sq ft – in residential building Tavistock II on Tregunter Path on Tuesday, arresting the owner, 60, and her sister, 72, and seizing a large quantity of evidence, including HK$10 million (US$1.27 million) in cash, share certificates and a car.

The younger sister allegedly controlled the syndicate, using the flat to take calls from customers seeking sex services. The elder sister is believed to have been her assistant.

Chief Inspector Paul Ng Yu-fan, from the bureau, said a website operated by the ring carried pictures of expat women who worked as prostitutes, promoting all-day high-quality escort and massage services for upwards of HK$7,000 per session, depending on the duration.

The website, which targeted foreign visitors to Hong Kong, claimed all the sex workers were students or models and their services were legal in the city.

Five other women – three suspected prostitutes from Venezuela and Russia and two domestic workers – were also arrested in the same operation.

Ng said the ring had been operating for more than a year and also controlled prostitutes from Eastern Europe and South America. Investigators are looking into whether it had links to triads.

On the same day, the bureau raided the office of another sex syndicate in a building on Kimberley Road in Tsim Sha Tsui, arresting a man and a woman who were said to be the masterminds.

A second man, a computer technician, was arrested in another building in Hung Hom.

Superintendent Simon Kwan King-pan said the trio had been operating a website advertising dozens of vice establishments in various parts of the city, including massage parlours, brothels and bathhouses, for about a year.